Introduction
In automatic lathe and CNC turning production, SUS303 is by far the most commonly used stainless steel bar stock. While SUS316 and SUS316F are also used, SUS303 dominates the turning shop floor.
Yet in practice, customer drawings frequently specify "SUS304" as the material. Why? Most likely because customers are more familiar with 304 as the generic stainless steel designation. However, for turned parts, specifying 304 makes machining significantly more difficult and costly — with no meaningful benefit for most applications.
This article compares the actual chemical compositions of SUS304 and SUS303 as used in CNC turning bar stock, explains why 303 is the correct choice for turned stainless parts, and clarifies the corrosion resistance trade-off.
Content authorized for publication by FULLERKREG.
Chemical Composition Comparison: SUS304 vs SUS303 (Turning Bar Stock)
SUS304 (as used in turning bar stock)
| Element | C | Mn | Si | P | S | Ni | Mo | Cu | Cr |
|---|---|---|---|---|---|---|---|---|---|
| Range | ≤ 0.025 | ≤ 1.78 | ≤ 0.3 | ≤ 0.027 | ≤ 0.02 | ≤ 8.1 | ≤ 0.13 | ≤ 1.96 | ≤ 18.2 |
SUS303 (as used in turning bar stock)
| Element | C | Mn | Si | P | S | Ni | Mo | Cu | Cr |
|---|---|---|---|---|---|---|---|---|---|
| Range | ≤ 0.04 | ≤ 2.08 | ≤ 0.3 | ≤ 0.027 | ≤ 1.76 | ≤ 8.14 | ≤ 0.13 | ≤ 1.96 | ≤ 18.02 |
Key Differences at a Glance
| Element | SUS304 (turning) | SUS303 (turning) | Purpose of difference |
|---|---|---|---|
| Sulfur (S) | ≤ 0.02% | ≤ 1.76% | S is the primary free-machining element — dramatically improves chip-breaking in SUS303 |
| Manganese (Mn) | ≤ 1.78% | ≤ 2.08% | Higher Mn in 303 improves machinability and chip formation |
| Carbon (C) | ≤ 0.025% | ≤ 0.04% | Slightly higher C in 303 contributes to machinability |
| Nickel (Ni) | ≤ 8.1% | ≤ 8.14% | Essentially identical — both are 18-8 austenitic grades |
| Chromium (Cr) | ≤ 18.2% | ≤ 18.02% | Essentially identical — primary corrosion resistance element |
The key insight: Comparing the two compositions, the most significant differences are in sulfur (S) and manganese (Mn) — both elevated in SUS303 specifically to improve machinability. The corrosion-governing elements (Ni and Cr) are nearly identical between the two grades.
Why SUS303 is the Correct Choice for Turned Parts
1. Machinability
SUS303's high sulfur content (up to 1.76% vs ≤ 0.02% in SUS304) creates sulfide inclusions that act as internal chip-breakers, enabling:
- Higher cutting speeds on automatic lathes and CNC turning centers
- Shorter, cleaner chip formation (no long stringy chips)
- Longer tool life
- Better surface finish
- Lower machining cost per part
Machining SUS304 on an automatic lathe is significantly more difficult and costly. There is no reason to accept this penalty when SUS303 delivers equivalent corrosion performance for most turned part applications.
2. Corrosion Resistance
Since Ni and Cr content are nearly identical in both grades, their base corrosion resistance is very similar. However, SUS303's higher sulfur content means:
- In standard conditions (without surface treatment), SUS303 performs slightly below SUS304 in salt spray testing
- The primary reason is the elevated S and Mn content, which can affect passive film uniformity
- Solution: When corrosion resistance is critical, SUS303 parts should undergo passivation treatment, which significantly improves salt spray performance and effectively closes the gap with SUS304
3. The Practical Conclusion
For stainless steel turned parts: specify SUS303, not SUS304. The machinability advantage is substantial; the corrosion resistance difference is minimal and can be eliminated by passivation when needed. Specifying SUS304 for turned parts adds machining cost and difficulty with no meaningful benefit for the vast majority of applications.
When to Actually Use SUS304 for Turned Parts
There are specific cases where SUS304 (or SUS304-grade material) is genuinely required over SUS303:
- Welded assemblies: SUS303's high sulfur content causes hot cracking in welds. If the turned part will be welded, use SUS304.
- Extreme corrosion environments: In highly aggressive media where even passivated SUS303 is insufficient, SUS304 or SUS316 may be required.
- Customer specification is non-negotiable: Some industries or customers mandate SUS304 by specification regardless of application.
For all other turned part applications, SUS303 is the correct and preferred material.
Related Material Guides
- SUS303 Free-Machining Stainless Steel — Full Chemical Composition Guide
- SUS304 Stainless Steel — Full Chemical Composition Guide
- SUS316 Stainless Steel — Full Chemical Composition Guide
- Common Bar Stock Materials for Automatic Lathes — Selection Guide
Content authorized for publication by FULLERKREG.